FIG. 3 is a functional block diagram of a conventional optical disk apparatus including a conventional speed detecting system.
This apparatus includes a spindle motor 3 for rotating an optical disk 1 having concentric or spiral tracks. The apparatus also includes an optical head 2 for reproducing information by radiating a beam spot on an information track of the disk 1. The apparatus further includes means 4-11 and the detecting system 21.
The means 4 is a head positioning means for moving the optical head 2 radially of the disk 1. The means 5 is a tracking error signal detector for generating a tracking error signal which represents the positional displacement of the beam spot on the disk 1. The means 6 is follow-up control means for constituting a follow-up control loop by feeding the tracking error signal back to the head positioning means 4 so that the beam spot follows a desired track. The means 7 is a track count pulse generator for forming, out of the tracking error signal, square waves (track count pulses) which cycle as tracks are traversed. The means 8 is a track counter. When the beam spot seeks a desired track, the number of tracks to be traversed is input as an initial value into this counter 8. Thereafter, the counter 8 subtracts in accordance with the track count pulses, and generates a target track drawing or pull-in timing signal after the remaining tracks are zero in number. The detecting system 21 is a period measurement type speed detecting system for detecting the relative speed of the beam spot with respect to the disk 1 by measuring the period during which a track is traversed. The means 9 is a target speed command means for commanding the relative speed of the head 2 in accordance with the value from the track counter 8. The means 10 is an error signal generator for outputting a difference signal based on signals from the speed detecting system 21 and target speed command means 9. The means 11 is a control loop switch. In order for the beam spot to follow a desired track, this switch 11 switches between the follow-up control loop that feeds the tracking error signal back to the head positioning means 4 and the speed control loop that feeds the output from the error signal generator 10 back to the positioning means 4, switching at the best timing for positioning on a target track, when seek starts, and when the value from the track counter 8 is zero.
The speed detecting system 21 will be described below in detail. Hereinafter, the relative speed of the beam spot means refers to the beam spot speed relative to the disk 1.
The speed detecting system 21 includes a reference clock generator 13' for generating a single clock having a frequency which can be as high as the highest frequency of the track count pulses. A counter 17' counts the output from the clock generator 13'. A latch 18' latches the count from the counter 17'. A set/reset controller 16' outputs a set signal for latching the value from the counter 17' for each cycle of the track count pulses and a reset signal for clearing, immediately after latching the value, the initial value of the counter 17'. A period/speed converter 20' outputs values proportional to the frequencies of the tracking error signal by converting the value latched by the latch 18'.
The output from the clock generator 13', which oscillates at a constant period, is latched by the latch 18' in accordance with the period of the tracking error signal detected by the detector 5 when the beam spot has traversed a track. When the period at which a track is traversed is short, that is to say, when the relative speed of the beam spot is high, a small value is latched. When the period at which a track is traversed is long, that is to say, when the relative speed of the spot is low, a large value is latched. Consequently, the latched values are inversely proportional to the speeds. Therefore, the speed detecting system 21 outputs the latched values after converting them into values proportional to the speeds by using the period/speed converter 20'.
The optical disk apparatus fitted with the speed detecting system 21 controls the speed of the beam spot in accordance with an optimum speed locus, and positions it on a target track.
A first problem with the conventional system is that the speed detector circuitry is large in scale and the production costs are high. The reason for this is that the period measurement type speed detector and the differentiation type speed detector are used together.
A second problem is that,the circuitry of the period measurement type speed detector is large in scale. The reason for this is that the count for a low speed movement area is huge, so that a multi-bit counter is needed, and the amount of operations of the period/speed converter is large.
A third problem is that the period measurement type speed detector needs to operate at high speed, and at the same time the power consumption increases. A reason for this is that the circuitry operates with a basic clock of a frequency which is some times as high as the maximum frequency of the track count pulses. Another reason is that the periods of the track count pulses in the high speed movement area are short, so that it is necessary to perform the operation of the period/speed converter at high speed.
A fourth problem is that, assuming that the speed detector is an F/V converter (which generates pulses of constant width with one edge of each track count pulse, passes the pulses through an integrating circuit, and thereafter equalizes them to be analog output), which may include a monostable multivibrator, the dynamic range and/or the accuracy of detection are/is not sufficient for speed detection for seek control and accurate positioning at high speed. The reason for this is that the traverse frequencies during a long distance seek, range over a wide band, and that the acceleration is large when a speed changes, so that the speed is not detected with accuracy.
A fifth problem is that it is difficult to match, with the differentiation type speed detector and the F/V converter, the values generated by the target speed command means and/or the period measurement type speed detector. The reason for this is that the target speed command means and the period measurement type speed detector operate with digital data, while the differentiation type speed detector and the F/V converter detect analog values and therefore necessitate adjustment for speed conversion.